Sun is a free and unlimited renewable source of energy. It can be converted directly to electricity by using p-n heterojunction solar cells (like silicon-based devices), electrochemical photovoltaic cells (EPC's) or dye-sensitized solar cells (DSSC's). EPC's are systems based on a junction between a semiconductor (p-type or n-type) and an electrolyte containing one redox couple; an auxiliary electrode completes the device. Owing to the built-in potential developed at the semiconductor/electrolyte interface, the photogenerated electrons and holes are separated and used to undergo oxidation and reduction reactions at the electrodes, respectively with the reduced and oxidized species of the redox couple. On the other hand, DSSC's are systems based on a junction between dye-chemisorbed nanocristalline TiO2 particles, deposited on a conductive glass substrate, and a non-aqueous electrolyte containing the I−/I3− redox couple; a platinum-coated conductive glass electrode completes the device. In such systems, the light absorption (by the dye molecules) and charge-carrier transport (in the conduction band of the semiconductor to the charge collector) processes are separated. Homogeneous oxidation of I− species serves to regenerate the photoexcited dye molecules whereas the heterogeneous reduction of I3− species takes place at the platinum-coated electrode.
There is extensive prior art on EPC's and DSSC's. However, one main issue still to resolve is to find a redox couple that is electrochemically stable, non-corrosive, with a high degree of reversibility and a high electropositive (in conjunction with n-type semiconductors) or electronegative (in conjunction with p-type semiconductors) potential, and colorless when used in concentrations allowing high electrolyte ionic conductivities.
I−/I3− is the most investigated redox couple for DSSC's. Cations may be alkali metals or organic cations containing quaternary ammonium groups such as dialkylimidazolium (Stathatos et al., Chem. Mater., 15, 1825 (2003)). The main limitations of this system are (i) the fact that it absorbs a significant part of the visible light of the solar spectrum when used in the concentration range giving reasonably good ionic conductivities (which leads to a decrease in the energy conversion efficiency); (ii) its too low redox potential (which limits the device photovoltage); (iii) its reactivity towards silver (which prevents the use of this metal as a current collector); and (iv) the high volatility of the electrolyte when usual organic solvents are employed (which causes an irreversible instability of the device).
Nusbaumer et al. in Chem. Eur. J., 9, 3756 (2003) studied alternative redox couples for DSSC's based on much more expensive cobalt complexes. Although the fact that these systems are less colored and possess more positive potential than the I−/I3− redox couple, the oxidized species (CoIII) may be reduced at the conductive glass acting as a substrate for the TiO2 particles, in which case the energy conversion efficiency is decreased. Moreover, regeneration of the dye molecules by the reduced species (CoII) (absolutely necessary to the operation of the device) may become more difficult due to association of the oxidized species (CoIII) with the sensitizer.
In EPC's, various redox couples dissolved in water were studied, such as Fe(CN)64−/Fe(CN)63−, I−/I3− Fe2+/Fe3+, S2−/Sn2−, Se2−/Sen2− and V2+/V3+, and devices exhibiting a good energy conversion efficiency were generally unstable under sustained white light illumination due to photocorrosion of the semiconductor electrode. The use of non-aqueous electrolytic media (liquid, gel or polymer) could eliminate the photocorrosion process, but in these cases the number of redox couples is very limited. For examples, the I−/I3− (Skotheim and Inganäs, J. Electrochem. Soc., 132, 2116 (1985)) and S2−/Sn2− (Vijh and Marsan, Bull. Electrochem., 5, 456 (1989)) redox couples were dissolved in polyethylene oxide (PEO) and modified PEO, respectively, and investigated in EPC's. In addition to the coloration and potential problems occurring with the I−/I3− couple, as mentioned above, the device stability has not been demonstrated. Regarding the S2−/Sn2− redox couple, the same problems were observed but in this case the stability under white light illumination has been reported.
A cesium thiolate (CsT)/disulfide (T2) redox couple, where T− stands for 5-mercapto-1-methyltetrazolate ion and T2 for the corresponding disulfide, was dissolved in modified PEO and studied in an EPC (Philias and Marsan, Electrochim. Acta, 44, 2915 (1999)). Its more positive potential than that of the S2−/Sn2− redox couple, its better dissociation in organic media including polymers (giving much more conductive electrolytes) and its much less intense coloration are responsible for the significant increase of the device energy conversion efficiency. Despite this improvement, the T−/T2 redox couple is quite electrochemically irreversible, with a difference between the anodic (Epa) and cathodic (Epc) peak potentials, symbolized as ΔEp, of 1.70 V at a platinum electrode (scanning speed of 100 mV/s), even when put in a more conductive gel electrolyte comprising 50 mM of T− and 5 mM of T2 dissolved in 80% DMF/DMSO (60/40) and incorporated in 20% poly(vinylidene fluoride), PVdF. Furthermore, its solubility is not very good in organic media.
Smith et al. in J. Org. Chem., 65, 8831 (2000) studied the redox hydrogen-bonded system formed from host-guest interactions with organic molecules that can bind through hydrogen bond and found that the redox couple of phenanthrenequinone (host) and urea (guest) undergoes a reversible one-electron reduction in aprotic medium. Collinson et al. gave more details about different kinds of redox-switched binding compounds (Collinson et al., Chem., soc., Rev. 31, 147-156, 2002). The articles of Smith et al. and Collinson et al. are hereby incorporated by reference.
Thus, based on prior art relative to redox couples for EPC's and DSSC'S, there are no redox couples permitting to considerably optimize the device energy conversion efficiency.
Therefore, new redox couples having improved properties with respect to the redox couples of the prior art would be highly desired. Moreover, redox couples permitting to avoid the drawbacks of the prior art are also highly desired. Finally, compositions or precursors that permit to easily prepare such redox couples would also highly be desired.